This invention relates generally to transmissions of the belt driven type and is more particularly related to the class of variable speed transmissions exemplified, in part, by the following patents: U.S. Pat. Nos. 3,479,908 to Kress et al.; 3,664,206 to Clauss, Jr.; 3,670,594 to Roper; 3,704,634 to Schrodt; 3,731,549 to Kaiser et al.; and 3,811,331 to Moogk.
Most of the transmissions of the patents enumerated above, as well as those of the rest of the prior art, utilize Vee-belts or variations thereof, such as link belts or chains, for transmitting power from one pulley to the other. This feature is in marked contrast to the thin, flat, flexible belt utilized in the present invention and which has numerous advantages to be pointed out more in detail hereinafter.
With present variable speed pulley drives of the type using Vee-belts, the latter are composed of a rubber composition and have a trapezoidal cross section, the belt transmitting rotary motion at one speed from a source of power, such as an engine or motor, to an output power shaft at another speed, the speed ratio being varied in a continuous fashion from a minimum to a maximum as dependent on the geometry of the belt and pulley system. The Vee-belt is compressed between smooth conical sheave sections in each of two pulleys by external axial forces on the sections to apply tension to the belt and friction between the sides of the Vee-belt and sheave sections to prevent slippage. In operation, a force unbalance caused by changes in the axial loading of the sheave sections causes the Vee-belt to change its radial positions in the two pulleys until a force balance is achieved or a limit range stop is reached. For a large transmitted torque the required axial force exerted on the sheaves results in a large compressive load on the Vee-belt, which requires the belt to have a substantial thickness to prevent axial collapse or failure of the belt. The increase in thickness increases its centrifugal force and causes higher belt tension loads. Also, as the belt thickness increases, pulley size must be increased due to higher stress loads at a given design minimum pulley radius. Further, the typical Vee-belt must continuously pull out from the compressive sheave load on leaving the pulley, which results in significant friction losses and belt fatigue, affecting overall efficiency and operating life. Consequently, although variable speed pulley drives have successfully used Vee-belts in a wide range of applications (industrial drives to snowmobiles and even automobiles) they have been severely limited in their power capabilities for more competitive smaller size equipment.